Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head "flies" over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has led to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the transducing head must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the transducing head. This is known as a "crash" and can damage the transducing head and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the stability of the shaft and bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor, because of low damping. Thus, there has been a search for alternative bearing assemblies for use with high density magnetic storage discs.
One alternative bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as gas or a liquid or air provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. Typical lubricants include oil or ferromagnetic fluids. Fluid dynamic bearings spread the bearing interface over a large continuous surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, improved shock resistance and ruggedness is achieved with a hydrodynamic bearing. Also, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat runout.
However, some fluid dynamic bearing designs themselves suffer from disadvantages, including a low stiffness-to-power ratio and increased sensitivity of the bearing to external loads or shock.
A desirable solution to this problem would be to have the spindle motor attached to both the base and the top cover of the disc drive housing. This would increase overall drive performance. A motor attached at both ends is significantly stiffer than one held by only one end.
Typically, fluid dynamic motor designs provide no method for top cover attachment. The reason for this is that in order to have top cover attachment, the motor (i.e. the fluid bearing which separates the fixed and moving parts) would need to be opened on both ends. Opening a motor at both ends greatly increases the risk of oil or fluid leakage out of the fluid dynamic bearing. This leakage among other things is caused by small differences in net flow rate created by differing pumping pressures in the bearing. If all of the flows and pressures within the bearing are not carefully balanced, a net pressure rise toward one or both ends may force fluid out through the capillary seal. Balancing the flow rates and pressures in conventional, known fluid bearing designs is difficult because the flow rates created by the pumping grooves are a function of the gaps defined in the fluid dynamic bearing, and the gaps, in turn, are a function of parts tolerances. Thus, a need exists for a new approach to the design of a fluid dynamic bearing based motor and especially the seals used to retain fluid in the motor.
As presently designed, fluid dynamic bearing motors seal the open end, whether it is just one end or both ends, using a capillary seal which simply comprises two relatively angled surfaces at the end of the gap with the seal being formed from one surface angled relative to the other. The problem with such conventional capillary seals is that they depend entirely on surface tension to maintain or draw fluid back into the motor. When used in pairs, as in a motor having an FDB which is open at both ends, such as a top cover attached motor, such a pair of capillary seals can be categorized as a pull-pull system; when there is more oil in one end than the other, the capillary seal with the lesser amount of oil pulls harder than the opposing capillary seal, and restores equilibrium. However, such capillary seals have been shown to be quite weak and have low volume. The problem with this low stiffness in a motor open at two ends is that only a small pressure imbalance in the motor can overcome the low seal stiffness, and cause oil to be lost with larger seal volume, the seal reservoir can store oil for equalization purposes.